1. Technical Field
The present disclosure relates to a polarization image processing apparatus that performs a polarization illumination process and a polarization imaging process on a transparent or translucent object.
2. Description of the Related Art
Technologies for detecting raindrops on a windshield of a vehicle with an image sensor have been proposed. If the raindrops on the windshield can be optically detected, the operation of wipers can be automatically controlled. However, since glass and raindrops are both transparent, it is difficult to detect the raindrops on the basis of color and brightness of objects.
Japanese Unexamined Patent Application Publication No. 2013-117519 discloses a technology for detecting the raindrops on the windshield by polarization imaging. With this technology, the windshield is irradiated with non-polarized infrared light emitted from the inside of the vehicle, and an image of the raindrops on the outer surface of the windshield is captured by an on-board camera. The camera includes an image sensor including an infrared narrow band filter made of a photonic crystal and a polarizing filter made of a metal wire grid. Image processing is performed to separate the image of the raindrops on the windshield from an image of the scene outside the vehicle by polarization imaging.
With the technology described in Japanese Unexamined Patent Application Publication No. 2013-117519, the windshield is irradiated with non-polarized illumination light. Therefore, it is necessary that an optical axis of the illumination light and an optical axis of the imaging system form the Brewster's angle. The inclination and curvature of the windshield vary depending on the type of the vehicle. Therefore, the angle of the transmission axis of the polarizing filter included in the image sensor and the installation position of the camera need to be adjusted in accordance with the type of the vehicle.